The fault with this is that unless you use session unique session ids to track visitors on your website and store these in the log file you have to make assumptions:
If I see the client IP is a.b.c.d and the next hit is from a.b.c.d then he must be the same visitor. (oops what about proxy servers? AOL? MSN? woopsie.)

If I see a delay of X minutes between hits then I am going to count this client IP as a new visitor, otherwise it is the same session and the session length is <time of last hit> - <time of first hit in session> long.

Depending on what level of accuracy you are looking for (and what percentage of your hits come from proxies) looking at the log files for this information just gives you faulty info.